Web-based social networks are emerging as the top applications on the Internet. With this immense popularity, many of the shortcomings of the current social network deployments are also coming to light. One of the glaring problems with existing web-based social networks is trust management. In this paper, we focus on trust modeling in social networks. Another allied issue that is not considered here is using trust in managing the activities within the social network. We introduce a gravity-based model for estimating trust. We present the complete model along with the trust computation algorithms. We present initial results from a simulation study that investigates the feasibility of the proposed scheme.